Writing instrument



FIG. 1 is a front perspective view of an ornamental design for a writing instrument according to the present invention;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a left side elevational view thereof;

FIG. 5 is a right side elevational view thereof;

FIG. 6 is a bottom plan view thereof; and,

FIG. 7 is a top plan view rotated 180 degrees from FIG. 6.

The broken lines are included to show features that form no part of the claimed design. 

CLAIM The ornamental design for a writing instrument, as shown and described. 